In recent years, III/V nitride materials, mainly GaN, InGaN, and AlGaN, have received much attention as semiconductor materials. Thanks to their continuously variable direct band gap from 1.9 to 6.2 eV, excellent physical and chemical stability, and high saturation electron mobility, the III/V nitride materials are the most preferred materials for optoelectronic devices such as laser devices and light-emitting diodes.
Due to the limitation in the growth technologies of GaN, however, large area of GaN materials are mostly grown on sapphire substrates. Although the GaN grown on a sapphire substrate has high quality and wide applications, the GaN based semiconductor devices are largely limited by the non electro-conductivity and poor thermal-conductivity of the sapphires. In order to avoid such disadvantages, methods have been invented to replace the sapphires substrate, after the growth of GaN based devices on sapphires, with high thermal-conductivity and high electro-conductivity materials such as Si, Cu, or the like. A commonly applied method for the removal of sapphire is laser lift-off technology.
The laser lift-off technology involves irradiating the GaN layer through the sapphire substrate at the interface between the sapphire and the GaN layer with a laser source having energy less than the band gap of the sapphire but larger than the band gap of GaN. As a result, the GaN absorbs the laser energy and yields high temperature. The GaN material at the interface is decomposed into gallium and nitrogen gas, which leads to the separation of the GaN layer and the sapphire substrate.
Conventional laser lift-off technologies use large laser-spots (having circumference larger than 1000 micrometers) to scan chip by chip (i.e. die) to achieve lift-off and separation of the GaN based device from the sapphire substrate. Such large laser-spot lift-off technologies include several disadvantages: because of the large fluctuation of energy on the edge of the laser-spot, stress is highly concentrated on the edge, resulting in that GaN at the edge of the laser-spot is seriously damaged, as shown in FIG. 1. The depth of the damage can vary from several tenths of micrometer to several micrometers, which is unavoidable. This disadvantage seriously limits the process for laser lift-off of GaN based devices.
The conventional laser lift-off process includes approximately the following steps:
(1) A GaN based epitaxial wafer is grown on a sapphire substrate;
(2) The epitaxial wafer with the sapphire substrate is made into GaN based separated device cells;
(3) Other thermal—conductive and electro-conductive substrates are electroplated or bonded;
(4) The sapphire substrate is removed by a laser lift-off method.
In the above mentioned process, in order to avoid the large laser-spot edge damage to the GaN based devices (which generally high power devices in dimensions of millimeters and power devices in dimensions of micrometers), the most adopted method directly covers one or more GaN based device cells, and to position the edge of the laser spot in passages between GaN based device cells to avoid laser-spot edge damages as much as possible. The above described also include several drawbacks: (1) the area of the laser spot must be accurately adjusted in accordance with the device size; (2) the position of the laser spot needs to be repeatedly aimed to ensure that the edge of the laser spot lay in the passages between GaN device cells; (3) a real-time video track detection is required to monitor position of the laser spot. When the edges of the laser spot are found to deviate onto the GaN devices, the operation must be immediately stopped for recalibration and correction. The above described issues are major obstacles to the application of laser lift-off technologies in large-scale device manufacturing, can significantly complicate the process and reduce production efficiency, and can increase the failure rate of the device (deviations in laser aiming or laser scanning can aggravate damages at the edge of the laser spot).